This invention relates to the use of a digital screen in the making of halftone copies of continuous tone documents.
In the past halftones were produced mainly by process camera contact screening methods. In modern graphics processing, electro-optical systems employing digital screens and laser scanning methods are also used. One such system is disclosed, for example, in U.S. Pat. No. 4,080,634 issued to William F. Schreiber on Mar. 21, 1978. The disclosure made by this patent is incorporated herein by reference. In that system an original document is scanned to produce video signals. The halftone screening is then performed by comparing the video signals with a succession of non-random dot reference signals, stored in digital form, representing the desired halftone screening pattern. Output circuitry then generates halftone recording signals, each of which is based upon the result of comparing the video and reference signals.
Non-random screens used in the system disclosed in the above-mentioned patent are quite suitable in graphically reproducing numerous details of the original document. However, certain other details are not reproduced as satisfactorily with non-random screens as with random screens. One system employing random screens (and a random number generator in generating the random screen) is disclosed, for example, in U.S. Pat. No. 4,040,094 issued to inventors Robert Charles Everett, et. al on Aug. 2, 1977. A problem with the use of such random-numbergenerated screens, however, is that since by being random they produce screen lines that are successively different, they tend to limit the dot size used in producing the halftone image and tend to "break up" desired repetitive patterns in the reproduced (recorded) image.
What is needed, therefore, is a random screen which is capable of meeting the requirements (e.g., different minimum dot sizes) of various printing processes which could be incorporated into a halftone reproduction device for providing (in addition to the effect produced by non-random screens) a random screening effect, yet permit the generation of repetitive patterns, thereby not limiting the dot size.